In recent years the art of plating on plastics has made major advances. The old methods had provided little or no adhesion between metal and the plastic so that the composite article lacked strength and durability. The newer techniques have provided good adhesion, from 4 to 20 pounds per inch when measured by the peel test. These newer techniques have used chromic acid to oxidize the smooth, non-polar, non-wettable plastic surface. This oxidative attack on the plastic surface should produce a polar, microporous, wettable surface. In the prior art illustrated in U.S. Pat. No. 3,445,350; U.S. Pat. No. 3,437,507; U.S. Pat. No. 3,533,828; U.S. Pat. No. 3,625,758; and the commonly-assigned copending application, Ser. No. 20,106, filed Mar. 16, 1970 now abandoned, the disclosures of which are incorporated herein by reference, all successful methods of producing this surface used chromic acid. Chromic acid has some serious drawbacks:
L. Chromium compounds are ecologically harmful because they are extremely toxic and the waste and rinse waters are difficult to treat properly;
2. Chromic acid residues on the plastic surface inhibit the electroless deposition processes used to deposit the initial metal film on the plastic article;
3. Chromic solutions are extremely difficult to rinse off which makes adequate rinsing before metallizing and treatment of waste water more difficult.
For many years those skilled in the art have attempted to replace chromic acid by another oxidizing agent in these processes. The usual material proposed to replace chromic acid has been potassium permanganate (Stahl et al., U.S. Pat. No. 3,625,758 suggest using highly acidic permanganate solutions). However, those attempting to use permanganate have not been successful. First, the surface treatment of the plastic would sometimes yield good adhesion and sometimes yield poor adhesion under identical treatment conditions. Second, acid permanganate solutions were notoriously unstable, had a short life and rapidly decomposed to manganese dioxide.
It has now been discovered that stable, highly active solutions for the treatment of resinous surfaces will be provided if the molar ratio of manganate ion (MnO.sub.4).sup.-- to permanganate ion (MnO.sub.4).sup.- is not allowed to exceed about 1.2 and if the compositions are controlled with, e.g., pH adjustors or buffers, to provide a pH in the range of 11 to 13.